Method for improving dent-resistance of a steel plate

ABSTRACT

The present invention relates to a method for enhancing a dent-resistant property of an outer panel of a vehicle. Pre-straining is performed to a steel plate by applying a certain reduction rate during a skin pass mill process performed for enhancing a density of a steel plate and adjusting a shape of the same, so that it becomes possible to enhance yield strength of a steel plate and to obtain an excellent forming property. The present invention is basically directed to fabricating an outer panel material such as a roof, a door, a hood, etc. of an excellent vehicle material having an excellent forming property and dent-resistant property. Therefore, it is possible to decrease a thickness of a vehicle material based on an enhanced strength of an outer panel of a vehicle. The present invention may contribute to a government environment regulation counter measurement and a combustion ratio of engine.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of Korean Application No. 10-2003-0049099, filed on Jul. 18, 2003, the disclosure of which is incorporated fully herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for improving dent-resistance of a steel plate by prestraining the steel plate through a skin pass mill process where a certain reduction rate is provided.

BACKGROUND OF THE INVENTION

In general, an outer panel of a vehicle requires improved processability for fabricating a certain part into a desired shape after a press forming process. the outer panel may also require an improved dent-resistant property. An outer panel of a vehicle body is generally made of a cold-rolled steel plate having a high elongation rate and a large plastic deformation rate. A high yield strength as well as excellent processability for fabricating an outer panel such as a door, a hood, a panel, etc. may be desirable. By contrast, if yield strength of a steel plate is low, the steel plate can be easily dented even when slightly pressed by hands or other external forces.

A dent may indicate that a certain defect occurred on a surface of an outer panel due to the above reasons. The property of limiting a dent is referred to as dent-resistance. A steel plate should generally have high yield strength to enhance dent resistance of an outer panel of a vehicle. However, when yield strength of a steel plate is too high, a process defect may occur during a forming process before the steel plate is formed. In addition, a spring-back phenomenon may occur after the forming process. It may therefore become challenging to fabricate a certain part in a desired shape.

Methods for enhancing yield strength of an outer panel of a vehicle body, to enhance dent-resistance of an outer panel of a vehicle body are known from the related art. For instance, methods such as a method of improving the amount of components of a steel plate, a method of using a combined material fabricated by mixing heterogeneous materials with each other, and a method achieved by combining both the above methods are known.

For example, according to the Japanese patent laid-open No. Hei 8-35035, there is provided a dual-skin steel plate formed of an inner steel panel and a surface steel plate with a different amount of steel for enhancing dent-resistance, a fatigue property, a surface deformation resistance and a processability. In another example, according to the Japanese patent No. Hei 5-111976, there is provided a core plate of aluminum or an aluminum alloy. A construction for enhancing the entire dent-resistance is provided by conjugating a low carbon steel plate to both surfaces of the core plate. In yet another example, according to the Japanese patent No. Sho 58-37150, a steel having a ferrite-martensite is provided on a surface of the construction, and a cold-rolled steel plate having a soft steel tissue is provided in an inner side of the construction.

Nevertheless, in the above methods, a surface defect called as a stretcher strain may occur on a surface of a steel plate, and reduce the ability to accurately control the amount of components. Moreover, the processability by the products is not uniform.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to a method for improving the dent-resistance of a steel plate. It may be possible to enhance yield strength of a steel plate and obtain an excellent forming property in such a manner that a steel plate is pre-strained by providing a certain reduction rate during a skin pass mill process performed for a density and shape modification of the steel plate.

In one embodiment of the present invention, an outer panel comprising steel material such as a roof, a door, a hood, etc., having an excellent processability and a dent-resistant property is fabricated using the method of the present invention. Increasing strength of an outer panel of a vehicle may allow for decreasing the thickness of the material.

In another embodiment of the present invention, yield strength of a steel plate may be enhanced based on an increase in the degree of work-hardening by pre-straining the steel plate in a state of one axis tensile prior to a press process.

In yet another embodiment, a method for pre-straining is achieved based on a skin pass mill process in accordance with a reduction rate of 1.8%˜2.0% during fabrication of a cold-rolled steel plate.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned aspects and other features of the present invention will be explained in the following description, taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a graph of an increase of tensile strength based on an increase of a reduction rate during a skin pass mill process for dent-resistance enhancing method according to the present invention;

FIG. 2 is a graph of a forming limit showing an increase in forming property after an extension based on a method for enhancing dent-resistant property according to the present invention; and

FIG. 3 is a graph illustrating a relationship between a pre-strain ratio and a FLD in a method for enhancing dent-resistant property according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, such embodiments of the present invention are described in detail with reference to the accompanying drawings.

In the methods according to the present invention, there are provided a few methods such as a method of applying a pre-strain during a skin pass mill process for enhancing a forming property and a dent-resistant property of parts based on yield strength enhancement of a steel plate, and a method of enhancing a reduction rate during a skin pass mill process.

In order to enhance a dent-resistant property of a steel plate with a mechanical process, it may be required to increase work-hardening, namely, yield strength (YP), by providing a material with a pre-straining process before a press process is performed.

Applying a pre-strain to a material is directed to deforming the material before a press process is performed. The pre-straining process may be performed within a range of 5%.

Pre-straining processes are generally performed in a deep drawing state or one axis extending state for preventing any changes in thickness of a material to enhance a forming property and strength. The pre-straining process may be performed by a method of providing one axis tensile force to a material before a press process is performed, but it is not very feasible to apply it to the practical field due to many obstacles in constructing the required facilities.

In the present invention, a method for providing a pre-straining process to a material before a press process is performed, there is provided a method of adjusting a reduction rate during a skin pass mill process. In this manner, it may be possible to apply the method to existing production facilities without additional facilities and any difficulties in the facilities or construction.

A skin pass mill process is directed to a process last performed during a cold-rolled steel plate fabrication and is capable of adjusting a density of a steel plate and correcting a shape of the same.

The reduction rate of the skin pass mill process may be limited within a range of 0.5%˜1.2% to provide a permissible range of a product thickness and density.

It is known that the mechanical property of material may be enhanced when a reduction rate is increased, but the above principle may not be applied in the field due to a decrease of elongations and the above-described problems. So far, the researches concerning the above problems have not been successful, so that systematic researches have not been performed yet.

FIG. 1 is a graph showing an increase of tensile strength based on an increase in reduction rate during a skin pass mill process for a dent-resistance enhancing method according to the present invention.

According to one result, it was known that a tensile strength was increased by 1.3 kgf/mm² when an elongation of a skin pass mill was increased by 0.4%.

Despite the above test result, it was impossible to increase reduction rate of the skin pass mill above a certain range because increasing the tensile strength may decrease the elongation in a tensile test resulting in a decrease in a forming property.

As shown in FIG. 2, however, even though the elongation may be decreased after the pre-straining process on one axis extension, the forming property may be increased. FIG. 2 is a forming limit diagram showing a forming property after one axis extension.

In one embodiment of the present invention, in order to measure the changes of material characteristics while increasing the reduction rate during the skin pass mill, the material processed through a conventional skin pass mill process was applied with a reduction rate of 0.5%, 1%, and the result of the tests were compared. Table 1 shows the changes in the mechanical characteristics of SPRC 340-BH by the pre-straining process. TABLE 1 Yield Tensile Tensional/yield PreStrain strength strength Uniform EI Elongation strength ratio (%) (kgf/mm²) (kgf/mm²) (%) (%) r-value n-value (%) 0 23.63 20.54 20.54 39.38 1.73 0.19 64.5 0.5 26.31 18.35 18.35 37.63 1.78 0.17 71.2 1 27.46 17.84 17.84 37.45 1.73 0.16 75.9

According to the data presented in Table 1, when the reduction rate was increased by 0.5%, the elongation was decreased about 1.8%, and the tensile/yield strengths rate was increased about 7%. When the reduction rate was increased by 1%, the elongation was decreased by 1.9% as compared with the conventional material, and the tensile/yield strengths ratio was increased by 11%. As a result of the above test, it is known that the tensile/yield strengths ratio may exceed 70% before the parts are press-processed.

The results of the forming properties in other stress states after the pre-strain is performed are shown as a forming limit diagram. Since the pre-strain of the increase of the reduction rate is one axis extension, it may be expected that the forming property is increased despite the decrease of the elongation.

In the present invention, the pre-straining process may be performed using a conventional skin pass mill process, and the reduction rate may be maintained within a range of 1.8˜2.0% higher than the conventional limited range of 0.5˜1.2%. Therefore, it may be possible to enhance the dent-resistant property of an outer panel of a vehicle, and enhance the press forming property.

According to a result of the tests, when the reduction rate was increased by 1.8˜2.0%, the yield strength of the SPRC 340-BH (material for FLD test) was increased 27 kgf/mm². Additionally, about 4˜5(kgf/mm²) of the yield strength was increased with respect to a reduction rate of 1%. Moreover, an additional increase by 2˜3 kgf/mm² was obtained when Bake-hardening steel capable of increasing the yield strength was additionally used after the press process using the heat treatment effect during the coating process.

An adjustment of these exemplary values may be required because high yield strength before the press process may cause a deformation in an outer panel of a vehicle in accordance with the material. Therefore, in order to fabricate a product having an excellent processability and dent-resistant property, it may be needed to systemize the physical characteristics of each material for thereby properly adjusting the above two effects.

FIG. 3 is a graph showing a relationship between a pre-strain ratio and FLD in a dent-resistant property enhancing method according to the present invention.

As shown in FIG. 3, the FLD was obtained by applying different pre-strains. Results of the tensile test following the pre-straining process, indicated that the elongation was slightly decreased by about 3%, but results of the FLD test showed a forming limit in different deformation states, where the elongation was slightly increased from 1%.

Generally, it is acknowledged that if forming property is not decreased after pre-straining process, it is successful. As shown in the above tests, the forming property was slightly increased after one axis extension.

As described above, the present invention provides a method capable of enhancing yield strength of a steel plate in such a manner that a pre-straining process may be performed with respect to a steel plate based on a skin pass mill under a condition that a certain reduction rate is maintained. Therefore, it is possible to enhance a dent-resistant property of an outer panel of a vehicle, and enhance a forming property during a press process of a steel plate. 

1. A method for enhancing a dent-resistant property of a steel plate comprising: pre-straining a steel plate by tensile forces acting along one axis; performing a press process with respect to the steel plate; and enhancing yield strength of the steel plate based on an increase in the degree of work-hardening.
 2. A method according to claim 1, wherein pre-straining the steel plate comprises performing a skin pass mill process on the steel plate, and the skin pass process comprises a reduction rate between about 1.8% to about 2.0% when a cold-rolled steel plate is fabricated. 